Why use Aperture or Shutter Priority?

I am pretty much a beginner at photography and don't know where to put this post, so I thought General Discussion would be a good place for it. I have gotten tons of good info from the members of this group so I figured I would start here...anyway...

I have an E-PL2, my first camera that is not a simple P&S, and I was wondering why one would choose to use Aperture Priority or Shutter Priority. I always shoot in Manual mode so I can control both of those features(in addition to ISO), so what advantages would I reap by using Aperture or Shutter Priority?

On a basic level, if you only need to control the speed of the shot (i.e. if you have a moving subject that requires a minimum speed) or you only want to control the depth of area in focus (i.e. getting a blurred background) but want to leave the remaining control to the camera, you'd use one of these modes.

I'm no expert, but basically I think of it like this: with exposure compensation, you basically get the same number of controls as M mode when using A or S, but instead of choosing the second value absolutely, you choose it relative to a rough guess by your camera. So, for example, if you choose S mode, then you can set the shutter speed appropriate for the type of shot that you want (say, high speed for sports), then the camera guesses the appropriate aperture (@ 0 exposure compensation), which you can adjust up or down. The nice thing about is that if you know you'll want to adjust one between shots, you won't have the to touch the other much, if at all. Otherwise, you'd have to change both to keep the same exposure level.

Do you use the camera's light meter when using Manual Mode? If so, then you are doing the same thing as Aperture or Shutter priority. Most likely you set one before the other depending on whether you have a required shutter speed (for stopping motion or shake, perhaps) or if you have a required DOF (this is the priority factor). From there, you adjust the other (lesser priority) factor to match your meter reading, or where you would like to end up on the meter (ie, +0.3EV, +0.7EV, etc). In Aperture or Shutter Priority mode, the camera takes over the second part for you, and adjusts directly to the meter the way you want (the way you've adjusted your Exposure Compensation). It just reduces the same process by one step.

For most of us, ISO bumps come into play when neither of the first two factors will get you in the range you need, then a bump in ISO will bring you into a new shutter speed bracket. That doesn't change whether in a priority mode or manual mode. However, in Shutter Priority an Auto ISO can be very handy for once the lens reaches its maximum aperture the Auto ISO then takes over to obtain your required shutter speed. This is a good combination for quick action shooting, taking away the time needed to make manual adjustments. Another good use for Shutter Priority is shooting in the studio when you have a lot of strobe lights and want to keep at max sync speed. The camera will then stop down the aperture as much as needed to expose properly with that sync speed (assuming you can keep within your camera's limits for Exposure Compensation!).

Your full automatic runs with a series of pre-determined fstops and shutter speeds depending on the lighting conditions. This will give you a nice exposure for most conditions.

But what happens when you go, for instance, to an air show? Planes flying around at high speeds, bright sunlight? The pre-determined fstops and shutter speeds would give you a combination that stops down the lens significantly. But what you really need is a very fast shutter speed, preferably as fast as your camera can manage.

Alternatively, you want to take pictures of two kids. They've been persuaded to sit down together quietly, but only if they can sit behind each other. You want both to be in focus: you choose the aperture to ensure that if the camera focuses on the back kid, the front kid will also be in focus.

I always use aperture priority, so I can control depth of field. If the shutter speed is too fast or too slow for the aperture I want I may adjust ISO or the apeture if needed.

In automatic mode, most cameras will prioritize shutter speed over aperture to minimize blur from shake. Typically I am less concerned with speed, and more concerned with depth of field, so aperture priority is a better fit for me.

Theres also a handy simulator - it doesn't explain the difference but you can mess with the settings and get a feel for the interdependency between Shutter Speed, Aperture and the resulting image yourself.

Shutter priority for panning at motorsports (1/60s say, or much faster for crashes!)
shutter priority at air shows for prop planes and helicopters for rotor blur, but aperture priority for jets (f8 or as close to it for the really sweet spot on my particular lens)

Then manual for Studio work given the meter in the camera would be completely fooled by those lighting conditions!

Theres also a handy simulator - it doesn't explain the difference but you can mess with the settings and get a feel for the interdependency between Shutter Speed, Aperture and the resulting image yourself.